The present invention relates to data transmission to and from down hole equipment and in particular, though not exclusively, to an improved data communication system and a method of data transmission through a three phase power system between the sub-surface and a surface location.
“Down hole equipment” is understood to refer to any tool, equipment or instrument that is used in a wellbore.
Data needs to be transmitted between down-hole equipment and the surface for various reasons. For example, monitoring performance of motors/pumps; transmission of control signals for control of valves; measuring device orientation and position, and making physical measurements.
For motorised down hole equipment, such as an Electric Submersible Pump (ESP) motor system, data needs to be sent from below the equipment in a circuit that includes motor windings and the equipment's power cable which can be considered as a three phase power system. In such arrangements, as power cables are already present, there is the rationale that the cost of the solution of using these should be proportionately less than a solution where an appropriate length of communication cable is also supplied. It is also generally accepted that being able to maintain power on the down hole monitoring instrumentation when the main three phase power system is not powered up is needed, as this provides essential information in the event of pump shut downs or other major events in the well.
Thus these systems are challenging to design and operate to ensure data is successfully transmitted and an independent power supply is maintained at all times.
Due to the motor and power cable properties of a three phase power system, DC current based devices which are coupled to the power system using inductive couplings have been developed and are extensively used. Power is provided from a low current DC power supply at surface and data is transmitted to surface by modulating the current drawn from this supply.
Examples of digital and processor based devices are disclosed in U.S. Pat. No. 5,515,038; GB2283889 and U.S. Pat. No. 6,396,415. These systems utilise DC current injected onto the power signal and extracted through inductive Y-point couplings. These systems are all susceptible to failure when insulation on the power cable is lost or damaged, as any fault is in parallel with the independent power source, and the fault becomes another current modulation source thus causing signal integrity to be lost. These prior art systems are also typically either analogue in nature, thus introducing noise into the measurements or, where digital data is transmitted, it is at a very slow data rate.
AC based systems which make use of AC power and/or signal transmission have been developed to overcome these problems. However, these AC based systems introduce disadvantages of their own. A typical prior art AC based system is disclosed in U.S. Pat. No. 7,982,633 being a data communication system for use in down hole applications wherein electrical energy is supplied over a multiple-conductor power cable to an ESP motor assembly. A down hole unit is AC-coupled to the conductors of the power cable through the wye point of the ESP motor assembly. A surface unit is AC-coupled to the conductors of the power cable. Uplink communication of telemetry data occurs over an AC communication scheme supported by the down hole unit and the surface unit. Downlink communication of remote control command data occurs over a different AC communication scheme supported by the surface unit and the down hole unit. These AC communication schemes provide an independent supply of power to the down hole environment. All communication between the surface and down hole environment is accomplished through the power cable without the use of additional communication lines. Data communication is maintained in the event of a ground fault on the power cable.
The expressed intention of such prior art AC based systems is to operate when the insulation on the power cable is damaged or at least imperfect. However, a disadvantage of these systems is that when a fault in the ground insulation exists the load presented by the ESP power system may be excessively high. When such an excessive load fault exists it is necessary to completely disable the system in order to protect the instrument power system from the excessive power supply load and reduce loading on signal transmission. In U.S. Pat. No. 7,982,633 there is disclosed an arrangement where high pass filtering is used to remove the low frequency ESP motor power, typically around 25-60 Hz, applied across the down hole unit. In practice, however, the insulation fault will also pull the surface and down hole star points down toward the shorted phase. This in turn loads the downhole signal driver and consequently attenuates the recovered data signal. The level of attenuation may be sufficient to render the signal as irrecoverable and thus the system cannot function. It also loads the surface power supply and if the current loading is too high this may collapse also rendering the system inoperable.